Single-rail integrated circuit systems which include analog devices and which also employ only a single voltage power supply and ground return, typically require the generation of an on-chip mid-supply voltage for an analog ground (AGND) reference. One currently available method of generating the mid-rail voltage while maintaining a low AC impedance is to use large value polysilicon resistors as a voltage divider to set the half-supply voltage, and then using an operational amplifier configured as a voltage follower (i.e., having unity gain feedback) to buffer the AGND supply. With the unity gain buffer approach, however, significant trade-offs must be made between circuit stability, bandwidth and slew rate. At d.c., the closed-loop output impedance of an operational amplifier is equal to its d.c. open-loop output impedance (.about.1K.OMEGA. for a CMOS device) divided by the loop gain, which is typically on the order of 1.OMEGA.. At the unity gain frequency and beyond, however, the operational amplifier output impedance approximates the a.c. open-loop impedance which typically can range between 1-10.OMEGA. for a CMOS device. The result is that the mid-rail voltage supply generator will be slow to respond to frequencies beyond its unity gain bandwidth, such that high speed clock coupling and high frequency noise become a problem.
Since CMOS circuits are primarily capacitive in nature, the AGND (analog ground) output node of the operational amplifier will have a large amount of capacitance coupled to it, and therefore, for unity gain stability, the operational amplifier must be internally compensated which decreases its slewing capability. To increase slewing in turn requires more current, and thus more power dissipation. Finally, because the AGND voltage generator must drive a capacitive load, then for the bandwidth to remain relatively constant, the ratio of the transconductance g.sub.m of the operational amplifier input stage to the value of the compensation capacitor C.sub.c must remain constant, even as larger compensation capacitors are required. Therefore, the transconductance g.sub.m must also increase as larger values of the compensation capacitor are required. Each of these design modifications causes an increase in the physical size of the mid-supply generator and in the required supply current.
Thus, the need has arisen for an improved mid-rail voltage supply generator having good stability, bandwidth and slew rate, while at the same time being relatively small in physical size and requiring minimum supply current.